Conventional weigh batch blenders for the plastics industry incorporate a single, scale-mounted weigh hopper into which the different ingredient materials of a desired blend are sequentially fed from two or more screw feeders or other dispensing units. The feeding of the different ingredient materials is metered by a control system which responds to the incremental increases in the total weight of the weigh hopper as each ingredient material of the desired blend is added to the weigh hopper. After the control system has caused the desired weight of each ingredient material to be dispensed into the weigh hopper, the entire contents of the weigh hopper are dumped into a mixing chamber located below the weigh hopper. The dumping is usually effected by a pneumatically operated gate, such as a trap door, in the bottom of the weigh hopper. Motorized rotating mixing blades in the mixing chamber blend the ingredient materials together. Utilization equipment, such as extrusion or molding machines, draws the blended material, as required, by gravity through a port in the bottom of the mixing chamber.
One problem with conventional weigh batch blenders is the tendency of certain ingredient materials to segregate due to the action of the stirring blades, whereby non-uniform blends are delivered. The action of the mixing blades, intended to mix the ingredient materials, can, in fact, cause segregation of the ingredient materials. Color concentrate pellets, for example, might not mix uniformly with natural plastic resin pellets due to differing size, density or electrostatic properties.
Other shortcomings of conventional weigh batch blenders are excessive size, complexity of design and difficulty in cleaning. Because the weigh hopper is located above the mixing chamber, conventional weigh batch blenders are relatively large in height. The design of the motorized mixing blades, positioned in the mixing chamber, and the mechanisms by which the mixing blades are coupled to the source of power tend to be complex. Thorough cleaning is normally required to avoid contamination when changing recipes, particularly when changing colors. Because of the number of parts (e.g. weigh hopper, discharge gate and actuating mechanism, mixing chamber and mixing blades) which come into contact with the blended ingredient materials and the complexity in design cleaning can be difficult and time consuming even when the parts are designed to be removable.